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 Carbon Dioxide | Methane | Nitrous Oxide | Sulfur Dioxide
Carbon Dioxide
 The extent and speed at which humanity changes the climate will depend to a large extent on the rate at which society adds additional greenhouse gases to the atmosphere. If we continue to use today's technology, then as populations and the economy grow, emissions of CO2 will continue to increase. Until a few decades ago, energy consumption grew at about the same rate as the Gross National Product. After oil supplies were disrupted in 1973 and 1979, however, people made substantial efforts to decrease energy consumption, including better insulation, smaller cars, and more energy efficient appliances. As a result, U.S. emissions are only growing at about half the rate of economic growth. Although lower gasoline prices in the 1990s have led people to buy larger cars, many businesses are continuing to institute energy conservation measures.
Will the trend be toward increasing energy efficiency and greater use of non-fossil energy? In projecting future emissions of carbon dioxide, most studies use a set of six groups of scenarios proposed by the Intergovernmental Panel on Climate Change.  The lowest and highest of these scenarios both assume that the world's population will increase from 6 billion today to 8.6 billion by 2050, then decline to 7.1 billion in 2100. Under both scenarios, economic growth would be rapid, with global gross domestic product rising to $525-550 trillion by 2100, compared with 21 trillion in 1990. But the scenarios differ widely in their assumptions about how energy will be produced. Under the lowest scenario, 85 percent of primary energy in 2100 would be produced with non-fossil sources. Under the highest scenario, non-fossil sources would provide only 31 percent of primary energy in 2100. With these assumptions, annual global CO2 emissions from fossil fuel combustion would increase in the high scenario from 6 gigatons (GT, see glossary) in 1990 to 30.3 GT per year, but would drop to 4.3 GT by 2100 in the low scenario. Under a mid-range scenario, emissions would rise by about 13 GT by 2100.
Annual CO2 Emissions in the Four Groups of IPCC Scenarios
Source: IPCC 2000
All of these scenarios imply that CO2 concentrations in the atmosphere will increase throughout the next century. The IPCC scenarios produce CO2 concentrations ranging from 540 to 970 parts per million (ppp) by the year 2100, which is 90 to 250 percent higher than the concentration in the late 1700s. Today's concentration of approximately 370 ppm is about 95 ppm greater than the pre-industrial concentration. Thus, IPCC's scenarios imply that in the next century humanity might increase atmospheric CO2 by 5 to 10 times as much as we increased CO2 in the last century.
MethaneMethane emissions are not projected to rise as rapidly as carbon dioxide emissions. Rice paddies are a major source; and there is a limit to how much additional land can be used to grow rice. The IPCC scenarios show methane emissions in 2100 ranging from 236 to 889 million metric tons, compared with 310 million metric tons in 1990. Based on the range of emissions in the scenarios, IPCC estimates that future concentration of methane in the atmosphere would range from 1570 parts per billion (ppb) to 3730 ppb by 2100. The lower estimate is actually below today's level of roughly 1760 ppb; the higher figure is more than double today's concentration.
Nitrous OxideProjected emissions of nitrous oxide from fertilizer and other human activities in 2100 range from a 20 percent decrease to a 150 increase from today's emissions. The current concentration of 316 ppb is approximately 40 ppm greater than the pre-industrial concentration. The IPCC projects that by 2100, the concentration of nitrous oxide could be 354-460 ppb, 80-186 ppb greater than the pre-industrial concentration. Thus, in the next century, humanity may increase the greenhouse effect of nitrous oxide by 1-4 times the impact of the last century.
Sulfur DioxideThe IPCC estimates that sulfur dioxide emissions will drop by 30 to 70 percent by 2100, in line with the assumption that policies to reduce acid rain and other air pollution will reduce these emissions. In the United States, the Clean Air Act requires a 40% reduction. Unlike carbon dioxide and most greenhouse gases, sulfur dioxide is not accumulating in the atmosphere. Thus, a 70 percent decline in emissions would lead to a 70 percent decline in concentrations. As a result, the cooling impact of sulfate aerosols in the next century is likely to be less important relative to greenhouse gases than in the past.
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